Finding Far-flung Planets

Published: Summer 2004

“Boggle” might be the perfect name for the planet Notre Dame’s David Bennett and an international team of astronomers discovered recently. The distant world, estimated to be one-and-a-half times the size of Jupiter, is so far away from Earth and the means of its discovery is so novel that it boggles the mind.

The planet is the most distant from Earth ever discovered— 17,000 light years away, near the center of our galaxy—and the first to be found with a controversial technique called microlensing. Until now most of the more than 100 known planets were discovered by observing a telltale wobble in a star’s rotation caused by a nearby orbiting planet. The “wobble” technique works up to 170 light years away.

Microlensing is a clever idea based on an insight of Albert Einstein’s that allows scientists to use the universe itself as a kind of telescope. According to Einstein, massive objects, such as stars and planets, curve space and warp time in their vicinity. Thus light travels around the curved space in a way similar to how a magnifying glass bends light.

With that in mind, astronomers have been scanning the sky looking for instances where one star passes in front of another as seen from Earth. In that particular case the gravity of the foreground star can bend and focus the light of the background star. If the star has a planet revolving around it, a signature pattern of brightening and dimming will occur. The phenomenon is so rare —Einstein believed it never would be observed—that Bennett says astronomers had to look at hundreds of millions of stars to find the microlensing alignment of star and planet. The technique is controversial because it is difficult to verify since the microlensing event happens only once.

In summer 2003 a team of U.S. and Polish scientists using a telescope in Chile noticed a microlensing event in the constellation Sagittarius. When Bennett’s group, composed of scientists from the United States, New Zealand and Japan, looked at its data for the same event captured from a telescope in New Zealand the scientist detected the signature “brightness blips” indicating the presence of a planet.

Bennett’s group has a proposal pending with NASA to search for stars with planets using an orbiting wide-angle telescope half the size of the Hubble telescope. The new telescope, called the Microlensing Planet Finder (MPF), would have the ability to scan millions of stars every night looking for the telltale microlensing signature of a distant planet. If the MPF is funded by NASA, the Notre Dame research associate professor of physics will be the lead scientist for the project.